At present, a dual mode radio such as a mobile telephone (GSM/UMTS) or a mobile telephone/wireless LAN (GSM/WLAN) is commercially available; attention is focused on miniaturization of an RF filter section as a bottleneck to realize a multiband radio as a future compatible system. For this system, in addition to microminiaturization, high Q, variable filter, etc., becomes a key, but it is difficult to realize them under present circumstances. Then, in recent years, in integration, application of a filter using a minute electromechanical resonator as a multiband radio filter has been expected from the viewpoint of an affinity for an IC, etc.
As FIG. 11 shows an example, an electromechanical resonator in a related art is mainly made up of an oscillator 110 for selecting a resonance frequency, a support section 118 for supporting the oscillator 110 from a base, and an electrode for exciting and detecting the oscillator 110. A configuration of an electromechanical filter having a large number of electromechanical resonators arranged and mechanically joined in a joint section is also proposed (non-patent document 1).
FIG. 12 shows an example of the electromechanical filter. In this case, as the structure of the electromechanical filter, one joint section 134 exists for support sections 118 and 120 where at least two oscillators 110 and 112 are supported from bases 116. Further, to form a multistage filter, the number of the oscillators and the number of the joint sections are increased as required.
The electromechanical filter uses the following principle: First, when an RF signal is input to an excitation electrode 122, an electrostatic force occurs between gaps G of the oscillator 110 and the excitation electrode 122. At this time, the oscillator 110 and the oscillator 112 connected to the oscillator 110 via the joint section 134 largely oscillate only if the self resonant frequency of the oscillator and the frequency of the input RF signal. If the oscillator 112 oscillates, the capacity between the oscillator 112 and a detection electrode 124 changes and thus if a potential (DC voltage) is applied, an electric current is output from the detection electrode 124. Thus, the RF signal is output only when the self resonant frequency of the oscillator and the frequency of the RF signal, so that the device can be used as a frequency selective device, namely, a filter.
Non-patent document 1: Bannon, Clark, Nguyen “High Frequency Micromechanical IF Filters” Technical Digest, IEEE International Electron Device Meeting, San Francisco, Calif. 1996 pp. 773-776.